Process for treating chromite ores



Oct. 25, 1938. Q Q MAlER 2,133,998

PROCESS FCR TREATING CHROMITE ORES ATTORNEY.

Oct. 25, 1938. c.. G. MAIER 2,133,998

PROCESS FCR TREATING CHROMITE ORES Filed June 15, 1937 2 Sheets-Sheet 2 4 5 6 7 8 9 O 7mpera/ure /n /00 C.

(bar/e5 6. Maf/'er BY www ATTORNEY.

Patented Oct. 2.5, .1938

PATENT oFFicE raocEss Fon TaEATrNG cnaoMl'rE oaEs charles G. Maier, oakland, caux., assigner vto Great Western Electro-Chemical Company, a corporation of California Application June 15, 1937, Serial No. 148,322

16 Claims.

This invention enables chromite ores, especially those of grade lower than at present acceptable for the production of ferro alloys, to be successfully chlorinated in a continuously operated retort, with sublimation of virtually all of the elements forming volatile chlorides, including particularly chromium.

I have attempted to apply methods known to the art, and disclosed by patents, such as for example in the patents of dAdrian, (see U. S. Patent No. 1,434,385 of Nov.` '7, 1922), to a continuously operated vertical retort. I found that the volatile chlorides could not successfully be formed and volatilized by observing the conditions specified and known to previous workers, either in these patents, or others `covering the well known interaction of chlorine with oxidic ores and reducing agents. After a thorough investigation of the conditions of volatility of the chlorides of chromium, I discovered how to attain the desired objective of subliming the volatile constituents during, chlorination, in a continuously operated countercurrent retort.

In order to accomplish the continuous chlorination and volatilization of chrome ores, I have found it desirable to control the chemical conditions dened by a series of intricate chemical interrelationships between the several chlorides of chromium, and at the same time to control the physical conditions under which the chlorination 1., proceeds. I disclose these chemical conditions in this application, and the equally important and interrelated physical yconditions are made the basis `of a copending application.

ditions of dAdrian in a continuous retort furnace, a chromite ore of a type common to domestic Ideposits `was utilized, having the following analysis:

The fact that the total of the analyses is 100.8 rather than 100% is due to unavoidable errors inthe analysis of the various constituents, but has no practical significance for the present pur- In the early attempts made to utilize the con- This ore, ground to pass a hundred mesh screen, was intimately mixed with 12.25% of its weight in fine carbon (carbon black) and a small quantity of tarry, matter to serve as binder, and was pressed into pellets. An attempt was made to pass these pellets continuously through a vertical retort heated upwards to 1100 C., and there contacted with chlorine at a variety of temperatures. No continuous operation of the retort was possible. Incipient reaction began near 600 C., and some ferric chloride was distilled. Near 800 C. the reaction Was vigorous, the gas issuing from the retort was mostly CO2, some iron chloride and a little chromic chloride were produced, but in avery short time after admitting the chlorine stream, it became impossible to keep the charge in motion, and the back pressure on the chlorine feed line became excessive. On opening the retort, the entire column was found to be choked with a closely knit mass of chromic chloride crystals. It was apparent that the temperature was insuilicient to volatilize chromic chloride. The retort was again set up, and the temperature raised to 900-l000 C. A very vigorous primary reaction occurred, but the column was choked in even a shorter time than before. Upon opening the retort it was found to be completely plugged with chromous chloride. Virtually no chromicl chloride had been distilled.. It was apparent that simply raising the temperature to increase the volatility of the chromium chloride,

' did not solve'the dimculty.

An intensive laboratory investigation of the conditions of volatility of the chromium chlorides revealed the following facts:

l. Chromous chloride (CrCla) was found to have a melting point of 815 C., at which temperature the vapor pressure is 0.0007 atmosphere. The vapor pressures at 900, 1000, and 1100 C. are .004, .024, and 0.103 atmospheres, and the normal boiling point' 1302 C. At temperatures where chlorine begins to react readily with mixtures of carbon and chromite, a small but definite vapor pressure exists.

2. It was found impossible to determine directly the vapor pressure of chromic chloride (CrCla), since this material has an appreciable dissociation pressure as low as 700 C., lwith chromous-chloride and chlorine as primary products. By indirect methods, it was possible to determinethe thermodynamic vapor pressures of chromic chloride, as follows, at 600, 700, 800, and 900 C., .00007, .002, .039, and 0.396 atmosphere, respectively. 'Ihe normal sublimation temperature is 947 C.

The "thermodynamic vapor pressure of CrCh was secured by measuring the actual volatility at a given temperature and correcting this for secondary reactions which cannot be prevented at these temperatures and which are/discussed in sections 3 and 4 hereinafter. f

3. 'I'he volatility of chromic chloride in a chlorine atmosphere was foundto. be much greater than in an inert atmosphere such as nitrogen. This was proved to be due to the formation of a new chloride of chromium, not previously described in technical literature, .namely chromium tetrachloride (CrCll). 'Ihis material exists only as a gas at high temperatures, and upon cooling reverts to chromium trichloride and chlorine. The equilibrium constant for the reaction was. found to be at 600, '700, and 800 C.. .ll 0.02922, and 0.364 respectively. Thus when the total pressure is 1 atmosphere, the partial pressure of chromium tetrachloride in a chlorine atmosphere is .033, 0.157 and 0.448 of an atmosphere respectively. 'I'hese quantities are slightly less than the normal vapor pressure of chromic chloride in the lower part of the temperature range, ybut become more than ten times as great at 800.

4. 'I'he partial pressures of chlorine which would prevent the primary dissociation of chromic chloride. in a system containing condensed CrClr were determined as-follows, at 700, 800, 900, 1000 C., 0.000002, .00008, .003, and .043 atmosphere, respectively.

Based on these investigations and findings, I have devised a successful continuous chlorination and sublimation process which depends on maintaining the following conditions:

(a) The disposition of the chromite-carbon mixture in a vertical countercurrent retort is maintained in a physical condition, in accordance with my copending application, Serial No. 148,321 of June 15, 1937, permitting a definite and controlled excess of chlorine to be maintained at all times. Briefly, my copending case contemplates, among other things, disposing finely divided ore in a thin bed during the reaction, a bed not exceeding 3 m. m. in thickness. Such a bed is lconveniently provided by coating carrier particles of large mass with the fine ore by tumbling or otherwise coating the particles with the ore.

I have found experimentally that a partial pressure of chlorine of substantially10% is suitable for this purpose. It will be realized that virtually pure chlorine is admitted to the retort, but that the progress of the reaction absorbs chlorine, and produces carbon dioxide according, to the well known reactions:

that to achieve this necessary condition in a conaisance tinuous countercurrent unit it is desirable to practice the disclosure of my copending application.

In this way I completely prevent the harmful dissociation of chromic to chromous chloride at all temperatures suitable for sublimation of the chromium chlorides, preventing stoppage of the flow of gas and of passage of ore because of the accumulation oi' liquid CrClz, and at the same time I enhance the volatility of the chromium chlorides by causing appreciable concentrations of chromium tetrachloride to be supported by the chlorine partial pressure in accordance with the results disclosed in section -3 above.,

(b) 'I'he vertical continuous retort beingprovided with means for removing the gases consisting of sublimed chlorides, carbon dioxide, and excess chlorine at an intermediate point of the heated zone, I adjust the rate of downward pas# sage of solids, and the application oi' either external or internal heat in such a way that the maximum temperature of the column is greatest at a point somewhat below the above mentioned outlet, and so that this exit is never at a temperature lower than 900 C. This temperature is slightly above the temperature at which solid chromic chloride is in equilibrium with a partial pressure of approximately 0.38 atmosphere of chromic chloride gas. The above mentioned dilution of the gas leaving the retort with CO: produced by the reaction limits the maximum concentration of chromic chloride vapor to near this value. By maintaining the condition as stated no solid CrCl: is stable', and it sublimes as rapidly as formed, whencethe stoppage of the column by unvaporized chromium chlorides is prevented, and continuous chlorination and sublimation assured.

Understanding f the above conditions (a) and (b) requires further explanation, 'which is best disclosed by further reference to the specific conditions applying when vore of the analysis given above is being treated. Under conditions suitable for chlorination and sublimation of the chromium chlorides, all oxidic materials in the ore with the exception of silica are chlorinated. The chlorides of iron, chromium and aluminum are volatilized, and the chlorides of magnesium and calcium remain as liquids with the solids of the charge. Taking a unit charge as 2000 grams, the following stoichiometric conditions hold.

Formula Perwt. 1n Gm ff Cmmumt wt. cent 2000 gms. nln

1m. o2 51. o 1032 e. 7s

1i. s4 19. o an s 4s 101. 94 11. s 23e 2. s2

4o. a2 11. 5 23o s. 7o

sa os o. s 1o o. 2s

The theoretical carbon used, equivalent to CO1 formed is:-

For constituent Amount Total tions apply to the minimum amount of excess' v Similarly, the mols of Ch required are:-

For constituent Amount Total @pag.3

41. 50 mols Ch or 2941 gms.

Finally the volatile chlorides are the equivalent Constituent Gaseous mols All gaseous chlorides CO; from above All gaseous constituents Actually, this will not oc- Thus primarily or 0.357 atmosphere. cur, because of side reactions. one obtains followed by 2CrCl3+Cl2=2CrCl4 (2) Since reaction (2) uses up chlorine, reaction (1) then proceeds further, and the consecutive reactions will continue until a balanced or equilibrium condition is reached. When this is complete a considerable portion oi.' the chromium will be in the form of CrCla, which is liquid and relatively non-volatile, whence the retort becomes clogged and inoperative.

The actual chlorine content of the equilibrium mixture totals near 900 C. only a few tenths percent, but it is not sufilcient for successful operation to maintain a. slight excess of chlorine over this, as a primary addition, since any excess chlorine added at ilrst merely increases the relative amounts of CrCli as compared to CrCla, and' being used up in forming CrCl4 does not prevent the formation of CrClz. This situation will be understood by reference to Fig. 2, which shows the relative amounts of CrCl4, expressed as percent of the total amount oi' volatile chromium chlorides present, when various excess amounts of chlorine are added. Each curve, A, B, C, D, refers to a speciiled amount of C12 found in the exit gas (CO2) after reaction, and after reversal of reaction (2) upon cooling and condensation. These curves end in the broken line E, which shows the temperature at which-condensation of CrClz begins.

Above and below the temperature at which the normal vapor pressure of CrCla is adequate to cause complete volatility of the chromium chloride when diluted with the amounts oi' CO: produced by the reaction, two different sets of condichlorine which will sufdce to prevent the formation of CrClz. These are illustrated in Fig. 1, which shows in curve A the amount of Ch present in the exit CO: when CrClz is just prevented from `forming in the presence of solid CrCla, and in curve B the amount present when the amount of CrCh formed is insuilicient to saturate the gas phase' at temperatures above the normal saturation temperature. It Vwill be apparent that these curves intersect at a temperature just below the normal saturation point for a partial pressure of 0.357 atmosphere, and show that the peak is at 5.5% Ch in the exit.k Below these curves chromous chloride is formed.

Since in a countercurrent continuous operation chlorine and ore-carbon mixtures must necessarily be in contact at a variety of temperatures at or near the operating temperature, it is clear that an amount of excess chlorine must be used corresponding to at least 5.5%. The amount can most readily be coniirmed by analysis of the exit gas after volatile chlorides are condensed, and it has been found inadvisable to approach too closely to the critical point in actual practice. It has been found that between 10% and 11% in the exit CO2 is a safe working limit, allowing a margin of safety for uncontrolled uctuations in the operating conditions and variations of ore treated. It is further apparent that the critical point, while largely controlled by the chemical interrelationships of the chlorides of chromium, is also l somewhat dependent on the composition of ore treated. 'I'he term chlorine excess of about 10% employed in some of the claims, means chlorine in excess of that required to chloridize all chloridizable ore constituents to normal chlorides.

Thus, in order to show the conditions under which the largest amounts of excess chlorine would be required, corresponding tothe severest operating conditions, the upper curves A and B' are plotted in Figure l to show the limitations when pure CrzOs, rather than chromite ore, is chlorinated. It will be noted that* the curves A and A', showing the conditions limiting the formation of CrClawhen solid CrCls is present, differ only slightly for the chlorination of ore as compared with the chlorination of pure CraOs. In this instance the partial pressure of CrCls in the gas is fixed by the vapor pressures of this material, and the change is produced by the slight dilution eect of the volatile chlorides and their equivalent CO2 acting upon the concentration of other constituents, except CrCla.

The curves B and B', for temperatures above the dew points of CrCla, differ more appreciably, because in this instance the concentrations are affected by stoichiometric relationships. It is clear, however, that no greater concentrations of CrCls could be obtained under an'y conditions of chlorinating oxldic chromite ores than corresponding to the concentrations equivalent to the action of C12 upon pure CrzOa, whence the peak at 10.7% may be taken as the upper limit under any conditions of operation, and for any ore. Since this point must be determined indirectly, and may be subject to slight error, the ilgure l0 to 11%, which coincides with the experimental limit found by actual operationof a continuous chlorination retort, is taken as the practical upper working limit. Thus, while, from the above it would seem that if an ore lean in CrzOa were treated, the chlorine excess might be reduced, such practice would approach a condition of increasing the operating risk with respect to retort stoppage.

This may be explained as follows: 'I'he iron content of the chromite is somewhat more readily chlorinated than the chromium and it is possible to have in a travelling column of ore various relationships between CrzO: and FeO corresponding to the degree to which chlorination has proceeded, and this situation is "equivalent of an increase of CrzOa in the ore. Thus, the only abso- 'lutely safe working conditions are those correspending to pure CrzOa, since all actual operating conditions must necessarily be less severe than for the pure material, and any enrichment of the ore in CrzOa due to preferential chlorination at some intermediate point in the chlorinating column cannot require conditions more severe than for CrzOa-alone.

Referring now to condition (b), it will be apparent from reference to Fig. 1 that the temperature at which condensation occurs is somewhat depressed when excess chlorine is used. It has been found that if the temperature of the gas leaving the retort is maintained no lower than 900-920 C., which will be some l0 to 15 degrees higher than the true condensation point when 10%-11% chlorine is maintained in the residual gas after condensation of chlorides when treating the specified ore, continuous operation is feasible, and a safe margin for operating requirements is provided.

There is obviously no exact critical upper limit of temperature when the proper excess of chlorine is used. While chromic chloride vapor in equilibrium with solid crystals would require approximately .01 atmosphere chlorine to prevent dissociation at the normal sublimation point (947 C.), the dilution of the gas phase with CO: from the reaction prevents saturation at temperatures above 900-920 C., which results in a continuouslylessening requirement for excess Clz at temperatures yabove this point. Nevertheless, reference to the curves of Fig. 2 shows that the amount of CrCl4 decreases with the temperature, and that to ensure the advantages of the volatility of this material it is desirable not to utilize temperatures above 1050 to l100 C. Such temperatures are also inadvisable because of enhanced reactivity with the silica content of the ores, and with the furnace walls.

Refelging to the stoichiometric quantities presented abbv, it will be clear that 10% Clz in the exit CO2 corresponds to 2.15 mols of C12 per 19.38 mols CO2, whence the total chlorine requirements are 41.50+2.15=43.65 mols, or 1.053 times the theoretical. While this gives the theoretical basis for control of chlorine admission, it is advisable to check the required condition periodically by analysis of the exit gas, especially if the composition of ore changes markedly,

It is advisable to maintain the carbon at the exact stoichiometric equivalent of the ore, since this has a somewhat deleterious effect upon the rate of chlorination of the last of the chromic oxide. It has been found advisable to use about 1.25 to 1.35 times the theoretical carbon addition, whereupon extractions of over 99% of the chromium are readily obtained, and the reaction rate holds up well even in the lower stages of the counter-current action.

In Figure 3 I have shown a diagrammatic representation of an apparatus which I have successfully employed. In this apparatus, a vertical shaft `i is provided. 'Ihe shaft is made of inert material as silica, and I have successfully used the material known as Vitreosil. At the upper end of the shaft an ore inlet 1 permits the introduction of the charge, preferably in the form of the aforementioned carrier particles Heating'v coated with the'ore-carbon mixture. means, indicated at 8, provided in the form of electrical resistance, is positioned about a portion of the shaft intermediate the ends thereof. The upper portion of the shaft I is continued by a metal extension I2. The joint between the shaft ,and extension is protected by a surrounding water cooling jacket 9. About 10% of the chlorine is introduced through an inlet Il placed at the top of the shaft, the remainder being introduced through an inlet l2 adjacent the bottom of the equipment. A thermocouple well Il extends downwardly into the shaft to below outlet i6 which provides the exit for Volatilized materials. Exit I8 is placed above the lower end of the heating zone, to insure that the Volatilized chlorides are removed. Usually the exit is placed about a quarter of the zone length above the lower end, for this insures good results. At thebase of the shaft another cooling section I1 is provided, to protect the Joint between shaft I and a metal base structure and cool the lmaterials which pass therethrough to be removed by discharge conveyers I8 incase Il at the bottom of the shaft.

Volatilized products pass over through exit Il into condenser 2|. The condenser is also of silica (usually Vitreosil). condensed in the condenser and scraper 22 permits volatilized chlorides condensing on the side of the condenser to be scraped out into the bottom of the equipment onto plate 2l from which they can be removed by another scraper 24 which serves to remove the condenser chlorides into passage 28. The material collected in the passage 26 is drawn by scraper 2l into receiver 21. Unvolatilized materiaL'dust ines and the like, pass into vessel 28 wherein the gas is filtered through filter bag 29, usually an asbestos filter. A shaker 3| supports the upper end of the bag and this is shaken occasionally so that the collected dust drops down into the receiver 21.

In Figure 4 I have shown the temperature gradient relative to the heighth of the retort. This shows the actual temperature existing in any portion of the equipment under ,constant operating conditions.

Having thus disclosed the chemical conditions necessary to maintain continuous counter-current chlorination and sublimation of chromite ores, Y

I claim:

1. A method of continuously chlorinating chromite ores which consists in maintaining an excess of chlorine suiilcient to ensure and to effect substantially complete chloridization of chloridizable constituents in said ore and to promote substantial increase of volatility of chromium chlorides by conversion of lower chlorides to chromium tetrachloride, and maintaining a residue of free chlorine sumcient to prevent the dissociation of said chlorides to the chromous condition while maintaining a temperature at which chromium chlorides sublime freely to effect substantially complete removal of chrome from said chromite ore as the trichloride.

2. In 'a method of continuously chlorinating chromite ores, the control of operation and the maintenance of continued passage, by adjusting the rate of countercurrent chlorine iiow with respect to the rate of passage of ore so that the exit gases, after substantially complete chlorination Volatilized chlorides are 30 of au cmoridizabie materiau in said ore and condensation Aof sublimed chlorides including substantially all chromium as chromic chloride, contain no less than 5.5% free chlorine, and are norvmally maintained at substantially 10% free chlorine.

3. A method of continuously chlorinating chromite ores in which chromite-carbon mixtures are contacted with chlorine in proportion not less than 1.053 times the theoretical stoichiometric amounts required to produce normal chlorides in said chromite ore, and in which the gases produced by reaction and sublimation are removed from the ore column at temperatures not less than 890 C.

4. In a continuous countercurrent chlorination of chromite ores, the process which supplies chlorine in amounts suilicient to produce substantial amounts of chromium tetrachloride, and suilcient excess to maintain not less than 10-11% free chlorine in the products of chlorination after substantially complete chlorination of said ore and condensation of sublimed chlorides including chromium as chromic chloride, to a moving column of chromite ore so disposed in a retort or furnace that the volatile and gaseous products are removed from the ore column at a point on the upstream side of the maximum temperature of an ore column; and at a temperature of not less than 900 C.

5. In the chlorination of a chromite ore, the steps of passing said ore countercurrent to a stream of chlorine while maintaining said ore during said passage at a temperature of between 900 C. and 1100 C. and maintaining a chlorine excess of about 10%.

6. In the chlorination of a chromite ore, the steps of passing said ore countercurrent to a stream of chlorine while maintaining said ore during said passage at a temperature of between 900 C. and 1100 C. and maintaining a chlorine excess of about 10%, said ore being carried as a thin iilm coating of finely divided ore on carrier particles relatively massive with respect to said finely divided ore.

'1. In the chlorination of a chromite ore, the steps of passing said ore, in the presence of about 1.25 the amount of carbon required to reduce oxides in said ore, countercurrent to a stream vof chlorine while maintaining said ore during said passage at a temperature of between 900 C. and 1100 C. and maintaining a chlorine excess of about 10%.

8. In the chlorination of achromite ore, the' steps of passing said ore, in the presence of about 1.25 the amount of carbon required to reduce oxides in said ore, countercurrent to a stream of chlorine while maintaining said ore during said passage at a temperature of between 900 C. and 1100 C. and maintaining a chlorine excess of about 10%, said ore being carried as a thin lm coating of finely divided ore on carrier particles relatively massive with respect to said fine ly divided ore.

9. A method of treating a chromite ore comprising maintaining a body of said ore at a temperature' of about 900 C. and above and in the presence of chlorine sumcient (l) to convert al1 chloridizable material in said ore to chlorides except chromium and (2) to convert and maintain substantially all chromium in said ore as chromic chloride and chromium tetrachloride at said temperature.

10. A method of treating a chromite ore comprising maintaining a body of said ore at a temperature of about 900 C. and above and in the presence ofchlorine suiilcient (l) to convert all chloridizable material in said ore to chlorides except chromium and (2) to convert and maintain substantially all chromium in said ore as chromic chloride and chromium tetrachloride at said temperature and cooling said chromic chloride and said chromium tetrachloride to ensure formation of (1) chromic chloride as the solid phase containing substantially all the chromium in said ore and (2) an exit gas containing about chlorine.

11. A process for continuously chlorinating volatile chlorides from a finely divided chromite ore comprising disposing said ore, mixed with a reducing agent in a substantially uniformly dispersed mass, in a thin porous bed while subjecting the so disposed ore to the action of chlorine, the ore-chlorine ratio being so adjusted that the coincidental requirements for (l) formation of equilibrium concentrations of CrCl4 and (2) prevention of dissociation of CrCla are simultaneous-v ly maintained in said mass.

12. A process forcontinuously chlorinating volatile chlorides from a finely divided chromite ore comprising disposing said ore, mixed with a reducing agent in a substantially uniformly dispersed mass, in a thin porous bed while subjecting the so disposed ore to the action of chlorine, the ore-chlorine ratio being so adjusted that at any instant there is present in said mass chlorine in excess of that required (1) to convert all chloridizable materials, except chromium, therein to the highest chloride thereof and (2) to convert the maximum amount of chromium to the tetrachloride at the temperature at which volatile chlorides are removed from said ore bed.

13. A process for chlorinating a finely divided chromite ore containing various metals in combined form e. g. iron, chrome, calcium and magnesium, said process comprising maintaining a mass of said ore mixed with a reducing agent in a free gas permeable condition during said process at a temperature of about 900 C. and in the i presence of chlorine substantially uniformly distributed through said mass and more than sufiicient to convert substantially all metals-in said ore to chlorides in form corresponding for each metal except chromium to the highest valence of the metal and to convert and vaporize substantially all chromium as chromic chloride and chromium tetrachloride, and cooling vaporized chlorides in the presence of an excess of chlorine to ensure continuance of chromic chloride as the solid phase of chromium.

14. In a process of chlorinating a chromite ore in which a column'is employed having an outlet for volatilized chlorides from said ore, the steps comprising supplying ore to establish and mainltain a substantially vertically falling stream thereof in said column, supplying chlorine to said column and to the ore stream therein while heating said column to establish a chlorination zone therein, and' regulating and adjusting the rate of ore supply and of chlorine supply to maintain said zone adjacent to said outlet.

15. In a process of chlorinating a chromite ore in which a column is employed having an outlet for volatilized chlorides from said ore, the steps comprising supplying ore to establish and maintain a substantially vertically falling stream thereof in said column, supplying chlorine to said column and to the ore stream therein while heating said column to establish a chlorination zone thereimand regulating and adjusting the rateo!` ore lsupplynand of chlorine supply, (1) 'to maintain said zone adjacent to said outlet (2) to simultaneously maintain in said zone the coincidental requirements for (a) formation of equilibrium concentrations of CrCh and (b) prevention of dissociation of CrCla.

16. A process of chlorinating in a column having an outlet a chromite ore containing various metals in combined form e. g. iron, chromium, calcium and magnesium, said vprocess comprising supplying ore particles to said column to fall substantially vertically down said column in a continuous stream asa thin illmcoating on carrier particles relatively massive with respect to said ore particles and :in' the presence -of about 1.25 times the amount of carbon required to reduce oxides in said ore, heating said column and supplying chlorine thereto to establish a chlorination zone in said column, and regulating and adjusting the rate ot ore supply and the rate ot chlorine supply (l) to maintain said zone adjacent to said outlet, (2) to convert all chloridlzable materials in said ore, except chromium, to the highest chloride thereof and (3) to convert the maximum amount of chromium to the tetrachloride at the temperature at` which volatile chlorides pass through said column outlet.

CHARLES G. MAIER. 

